There are many known apparatuses for pipe bending, each with its own particular method of use and each with its own limitations. Certain features are commonly considered to be unacceptable in a bent section of pipe. Kinking of the surface of the section is usually found on the inside of the bend. It is considered a major fault Wrinkling of the surface of this section is also usually found on the inside of a bend. Wrinkling is like small but repeated kinks and is often not acceptable. Distortion of the original shape of the cross-sectional profile in the form of a flattening found at the outside of the bend is sometimes acceptable if only minor by visual inspection. Marks, dents or bulges left in the surface of the section by the bending apparatus is usually considered a minor fault from a mechanical standpoint but may be very significant in a visual sense.
In recent times, pipe and RHS have been available in C350 grade steel, which is stronger than previously supplied steels. This grade of steel is work hardened when it is cold formed into pipe. It is also welded at the seam by electric resistance welding. The resulting pipe and RHS section is stronger and much more resistant to deformation. However bending is itself a form of controlled deformation. Further, sections are now offered with thinner wall thicknesses to give a section of similar strength lower weight than previous sections. While the strength, section thinness and lower weight may be advantagous to most users, problems involving all of the bending faults mentioned above are encountered when using conventional bending devices which were designed for sections produced by prior means.
Most existing pipe benders use a rigid, curved former which controls both the cross-sectional profile of the section and the radius of the bend. One simple type of bender users two fixed posts to apply a reaction force, while the inner former is forced into the pipe section between the posts to create the bend. The bend starts at the centre and progresses in both directions along the section as the former is progressively advanced between the posts. This type of bender is usually referred to as a "fixed post bender". Because the reaction points are widely spaced (to allow the former to pass between them) the force applied is relatively low allowing easy bending. However, high former contact pressure is desireable to prevent wrinkling of the bend, particularly with C350 grade sections which are in use today. Severe kinking is usually experienced with C350 grade pipe in conventional formers with a radius of bend of approximately three times the diameter of the pipe. There have been attempts to avoid this by increasing the former radius from three diameters to four diameters and by making the former tighter on the pipe, even to the point of having the pipe squeeze into the former. It is intended that this grip in the former will stop the pipe rising out of the former at the point of bend and allow a kink to form. These measures are successful on some types of pipe and in most cases kinking is not found while flattening and wrinkling are only minor.
However, many users do not like the swept bend appearance of a four diameter bend as it affects design from both a mechanical point of view as well as aesthetic considerations. Also, variations in pipe quality, bending technique and former specifications all cause serious bending faults.
A fairly recent variation of the fixed post bender allows the reaction points to be kept fairly close to the point of bend while also providing for them to move outward as bending progresses. This is done by mounting a roller on each of two pivotting arms. The arms move apart as the former is driven between them. However, this method still results in some problems with extra light wall sections where some wrinkling still occurs and outside flattening is noticeable.
Certain more effective and also more complex and expensive benders are referred to as draw benders. They are arranged to start the bend at a predetermined point and progressively bend this section around a former in one direction only. This is usually done by providing a fixed inside former, with one reaction point also fixed. Bending is achieved by a sliding or rolling outside former following an arc concentric with the inside former's shape. This allows the reaction point to be kept relatively close to the actual point of bend at all times and high former contact pressure is maintained. This tends to minimise wrinkling. However, with some extra light wall sections, wrinkling is still encountered with considerably flattening around the outside of the bend.
Draw benders are usually arranged to make a bend progressively in one direction from a start point, by engaging the pipe to be bent between a fixed reaction point, a fixed inside former and a movable outside member which is attached pivotally at the centre of the fixed former. Some variations use a fixed outside member and an inside former and clamping reaction member that rotate together pulling the pipe around the bend.
On fixed inside former types the outside member is usually a shaped roller that moves at a constant radius to the fixed inside former which has a curved groove to shape and support the pipe as it is bent. Sometimes the outside member is a flat roller with a shaped straight outside former to distribute the bending force over a length of the pipe to avoid deformation of the pipe surface by a concentrated point of contact from a roller alone.
To load the pipe into the bender ready for bending the pipe can be slid between the three members but this is very awkward, even impossible if a series of bends are to be made on one pipe close to each other. For ease of use the outside member is moved away from the former far enough to allow the pipe to be lowered to the plane of bending and moved into the former groove. Then the outside member must be moved closer to the former again for bending. After bending the outside member must be moved away again to unload the pipe.
This movement is usually achieved by mounting the outside member in a carriage which has its position adjusted along the radial arm by a screw thread and handle. This means can introduce two problems; that it is awkwardly slow to operate, and that it does not accurately bring the outside member to the same radial position for all bends in a series of bends that may be required to be the same. In some industrial bending apparatus the carriage is moved radially in relation to the former by a hydraulic cylinder or other linear actuator. This is very expensive to arrange and physically large, making such benders only suitable for industry producing large volume runs of products using bent pipe.